Joannie10's answer will work in countries which use '00' as their international access code. That includes many European countries, but not the USA.
The international access code, in the USA, is 011. That needs to be followed by the national code for the UK, which is 44. Lastly, the full UK number, but omitting the initial '0', should be added.
So the UK number 01234 567890 becomes
011 44 234 567890.